it's listed as a 10 speed Shimano HG500, 11-42 teeth.
Just replace with the same if your mate is happy with the gearing
Any 10 speed shimano patern cassette will fit
if you go for a more expensive model (eg. SLX) one it will be a little lighter but function exactly the same way. Although durability of the largest sprocket will be reduced as it'll be softer Aluminium over the HG500s steel sprocket.
If replacing it through wear you'll need to replace your chain as well. Any decent 10 speed chain will do fine. Again more expensive ones perform similarly but might be made from more expensive materials or have protective plating/coatings applied.
Basically if you're not bothered about weight and keep your chain clean, lubed and well looked after save yourself some cash and fit the cheapest. Being mid range (price & quality) There aren't really any properly poor quality 10 speed parts around.